1. Field of the Disclosure
The present disclosure relates to a technical field of IP multimedia subsystem (IMS), and more particularly to a method for implementing a voice call continuity (VCC) network handover in ringing/ring back tone (RBT) or call hold, a related VCC application server (VCC AS), and a user equipment (UE).
2. Description of the Related Art
An IP multimedia subsystem (IMS) is a subsystem proposed by the 3rd Generation Partnership Project (3GPP) in Release 5 (R5) for supporting IP multimedia services, and is characterized in that session initiation protocol (SIP) is adopted and that the IMS is irrelevant to the access manners. The IMS provides a universal service platform for future multimedia applications and is an important step in the evolution to all-IP network service providing systems.
R5 and Release 6 (R6) for IMS standards were derived in a 3GPP standard system. After R6 became basically stable in March, 2005, Release 7 (R7) was proposed by the 3GPP. R7 not only enhances the existing system functions, but also adds some completely new functions.
As IMS voice services and circuit-switched (CS) voice services may exist concurrently in a period of time, the handover between voice services is a problem in urgent need of solutions. A voice call continuity (VCC) function is a new function proposed in R7 for solving the problem.
As defined according to the above standards, VCC refers to voice call continuity, that is, the continuity of a call is maintained when the user switches between various access technologies. For example, when the user moves from a conventional 2G network (e.g., a global system for mobile communications or code division multiple access network) to a 3G network (e.g., a universal mobile telecommunications system or high-rate packet data network), the call continues, and meanwhile other 3G services such as high speed internet access services can be initiated.
Referring to FIG. 1, in order to support the VCC, when a user initiates or receives a call via a circuit switched (CS) network (i.e., a 2G network), all signaling has to enter an IMS network. A network element VCC application server (VCC AS) in the IMS network is configured to control a VCC handover. Further, a network element that enables the user to access the CS network via the IMS network is a media gateway control function (MGCF). The MGCF also controls a media gateway (MGW) to route bearer traffic.
Currently, the VCC handover is assumed to be performed with an ongoing call, and VCC handovers in ringing/ring back tone (RBT) (i.e., calling/called situation) and call hold are not taken into consideration. As a result, problems may occur. For example, when a user equipment (UE) leaves the coverage of an IMS hot spot and is in ringing/RBT or call hold, as the UE cannot perform a VCC network handover, a call drop may occur and affect the conversation quality of the users.